Prosthesis for tensile load-carrying tissue and method of manufacture

ABSTRACT

The inventive article is a ligament or tendon prosthesis having multiple longitudinally parallel strands of microporous expanded polytetrafluoroethylene, the individual strands having an average porosity greater than 30% in the areas requiring tissue ingrowth. Additionally, strand dimensions and microstructure are selected so that tissue can penetrate throughout. The prosthesis is formed from multiple loops of a single continuous filament. Densified eyelets are formed in the loop for mounting to bone. The strands are twisted 180° or arranged in a loose braid about the prosthesis axis for improved load distribution during bending of the prosthesis.

This is a division of application Ser. No. 07/568,625, filed Aug. 16,1990 now U.S. Pat. No. 5,049,155, which was a continuation of Ser. No.06/732,811 filed May 10, 1985, pending, which was a continuation of Ser.No. 06/415,565 filed Sep. 10, 1982 now U.S. Pat. No. Des. 277,899.

FIELD OF THE INVENTION

The inventive article described herein is a synthetic prosthesis forreplacement or repair of ligaments or tendons.

DESCRIPTION OF THE PRIOR ART

The generally accepted method of repair of ligaments and tendons isthrough the use of tissue transplanted to the defect site from elsewherein the body. This method of repair often fails due to a number offactors, including insufficient strength of the transplanted tissues,dependence of the transplanted tissue on revascularization forviability, and inadequate strength of attachment or fixation of thetransplanted tissue.

A great need exists for a prosthetic device to replace damaged ligamentsand tendons, and there have been a number of previous attempts atproviding such devices. However, there is no prosthesis today which iswidely accepted. Among the reasons for failure of prosthetic devices areinadequate tensile strength, lack of adequate fixation, deterioration ofthe device due to mechanical stresses, and deterioration of theprosthesis/tissue interface.

Previous method of attachment to bone and soft tissues which have beenattempted include:

U.S. Pat. Nos. 3,971,670, 4,127,902, 4,129,470, 3,992,725, and4,149,277. These patents teach attachment through tissue ingrowth intoporous surfaces of the prosthetic device.

U.S. Pat. Nos. 3,613,120, 3,545,008, and 4,209,859. These patents teachmethods of tissue attachment to porous fabrics with various methods ofmaintaining apposition to the repaired tissue.

U.S. Pat. Nos. 3,896,500, 3,953,896, 3,988,783, and 4,301,551. Thesepatents teach attachment to bone by means of rigid mechanical devicessuch as screws, threads or other devices.

SUMMARY OF THE INVENTION

In accordance with the inventions, as broadly described herein, theprosthesis is made up of multiple porous strands ofpolytetrafluoroethylene (PTFE) formed from concentric loops of acontinuous filament. Immediate postoperative attachment of the device isprovided by integral eyelets formed from adhered, gathered loop ends,which can be affixed directly to bony tissue. This initial attachment isaugmented and finally made redundant as tissue grows into the porousstrand material providing permanent attachment of the prosthesis.

To achieve the foregoing objects and in accordance with the presentinvention, as broadly described herein, the method for making a tensileload-bearing tissue prosthesis of the type having a plurality ofparallel longitudinally adjacent strands connected to at least oneeyelet, the eyelet being for the initial attachment of the prosthesis totensile force-applying bone tissue, comprises forming a plurality ofelongated concentric loops from a continuous filament of the desiredstrand material until the desired number of parallel strands areobtained, the concentric loops defining a projected elongated area, andgathering the loop ends at one elongated area end to form the eyelet,the method including the step of securing the gathered loop ends againstungathering.

Preferably, the method includes the further steps of imparting a twistto the loop strands about the longitudinal axis of the prosthesis.

This invention will be further understood by reference to the drawingswhich are given for illustration only and are not intended to limit thescope of the invention but which are to be read in conjunction with thespecifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating the procedure for measuring thecharacteristic interstitial dimension of the material in thephotomicrograph in FIG. 4.

FIG. 2 is a photomicrograph of the PTFE material used in theconstruction of the prosthesis of Example B;

FIG. 3 is a photomicrograph of a PTFE material having nodes that areless well defined compared to the material in FIG. 2;

FIG. 4 is a photomicrograph of the material of FIG. 3 laterallystretched to provide for measurement of characteristic interstitialdimension;

FIG. 5 shows a schematic perspective view of one prosthesis constructedin accordance with the present invention;

FIG. 6 depicts schematically the implantation of an anterior cruciateligament prosthesis not constructed in accordance with the presentinvention;

FIG. 7 depicts schematically a stage in one method of construction of aprosthesis of the present invention;

FIG. 8 shows a schematic perspective view of another prosthesisconstructed in accordance with the present invention;

FIG. 9 depicts schematically a stage in another method of constructionof a prosthesis of the present invention;

FIG. 10 depicts schematically a perspective view of yet anotherprosthesis constructed in accordance with the present invention; and

FIGS. 11A, B, and C depict the implantation into a knee joint of theprosthesis of FIG. 8 into a knee joint as an anterior cruciate ligamentprosthesis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventive article described herein is a synthetic prosthesis forreplacement or repair of ligaments or tendons. The prosthesis is made upof multiple strands of porous PTFE. The porosity of the strands ischaracterized by interconnecting void space throughout. Stranddimensions are small enough to permit tissue growth in and through theentire strand. The percent void space, or porosity, is greater than 30%,which allows mechanical attachment of tissue in the interstitial spacesof the prosthesis to provide sufficient attachment strength. This degreeof porosity is a requirement only for those sections of the device whichare intended to be anchored through tissue fixation. Porosity, as usedhere, is defined as; ##EQU1## where: ρ₂ =density of porous material

ρ₁ =density of solid PTFE making up the solid content of the porousmaterial. For PTFE which has never been sintered ρ₁ =2.3 gm/cm³ and formaterials which have been sintered a value of 2.2 gm/cm³ is used for ρ₁,although this value can actually vary somewhat depending on sinteringand cooling conditions.

Immediate postoperative attachment of the device is provided by eyeletswhich are attached directly to bony tissue. This initial attachment isaugmented and finally made redundant as tissue grows into the porousstrand material, providing permanent attachment of the prosthesis totissue. Tissue can easily grow between and among the strands since theyare not attached to each other nor held together tightly. However, thedepth to which tissue can grow into each strand is governed by thedimensions of the interconnected void corridors or pathways through theporous microstructure. The complex intercommunicating void space isformed by the solid PTFE matrix. In some cases, the matrix is made up oflarge solid nodes interconnected by long flexible, relatively inelasticfibrils. Although the nodes may present rigid inflexible structures toingrowing tissue, the fibrils can be bent and pushed aside bypenetrating tissue. Other microstructures of this invention have muchsmaller nodes which appear merely as connection points for the fibrils.In both cases, the strength of the fibrils in tension is very high, andalthough they can be bent by tissue, they cannot be stretchedsignificantly. The microstructures of this invention can becharacterized by a mean interstitial dimension which can be used topredict the depth of tissue ingrowth. Short fibril lengths impede andbottleneck tissue invasion. Thus, for porous strands having short fibrillengths, the overall strand dimension must itself be small enough sothat ingrowth and attachment will occur throughout the entire strand.

The methods used here to characterize the fibril length of a particularmicrostructure rely on visual examination of that microstructure.Photographs at a convenient magnification can be provided throughscanning electron microscopy or, in some cases, light microscopy. Themicroporous PTFE materials of this invention can vary sufficiently intheir microstructure so that different techniques of measuring thecharacteristic interstitial dimension must be used. Strand fibers suchas those made by the process described in Example B possess amicrostructure which can clearly be characterized by nodesinterconnected by fibrils. The characteristic interstitial dimension formaterials of this type can be determined through a direct measurement ofthe spacing between nodes. This measurement is taken along a line placedin the direction of strength orientation (FIG. 2). A large enough numberof measurements must be taken so that the node spacing is adequatelycharacterized. The mean node spacing thus provided is used tocharacterize the interstitial space and thereby predict the depth ofingrowth into that microstructure.

In strand material which has been manufactured by a stretching processsuch as is described in U.S. Pat. No. 3,962,153, or the products of U.S.Pat. No. 4,187,390, the nodes of PTFE can be smaller and much lessdefined. In highly stretched products made according to these patents,node spacing becomes very large and fibrils are packed together. Thesintering step in production of these materials causes the bundles offibrils to coalesce and form secondary attachment points. For thisreason, the microstructure of such materials is not readily apparenteven under magnification. In determining the characteristic interstitialdimension of these materials, it is necessary to measure the distancebetween fibril suspension points rather than measuring the fibril length(i.e., node spacing). The interstitial dimensions of these materials canbe observed if samples are prepared for microscopy by slightlystretching the material at right angles to its direction of strengthorientation. Upon stretching the sample 10% in the lateral direction,with the sample restrained from shrinking in the longitudinal direction,the points at which fibrils are connected become apparent undermicroscopic examination. The distance between fibril connections is thenmeasured at all obvious gaps created between fibril bundles. Thismeasurement is taken in the direction of strength orientation. As withthe method described previously for node spacing, the number ofmeasurements of fibril suspension distance must be sufficient tocharacterize interstitial dimensions of the microstructure.

FIG. 3 shows how material of this type appears without lateralstretching as compared to FIG. 4 which is a micrograph of the samematerial with 10% lateral stretching. This lateral stretching, which isused only to characterize the microstructure of the material, representsa temporary structural reorientation. A force placed on the material inthe longitudinal direction causes a return to the original lateraldimension and a restoration of the original microstructure. Aspreviously described, it is believed that the fibrils composing thismicrostructure are pushed aside by ingrowing tissue. The method ofmeasuring the characteristic interstitial dimension for materials ofthis type is shown in FIG. 1 relating to the photomicrograph of FIG. 4.Having once determined the characteristic interstitial dimension by thetechniques described, the proper strand dimensions can be determined.

The maximum strand thickness which would allow tissue penetrationthrough the entire strand is approximately two times the tissuepenetration depth. The maximum strand thickness, as used here, refers tothe appropriate minor cross-sectional dimension of a strand, e.g., thediameter of a strand of circular cross-section or the thickness of astrand or rectangular cross-section. In general, combinations ofcharacteristic interstitial void dimension and strand thickness lessthan the maximum strand thickness are preferred because they allowcomplete tissue penetration across the strand cross-section in a shortertime interval.

A major requirement for a successful ligament or tendon prosthesis isthat of adequate strength. In many situations prosthetic materials usedto replace these natural structures are subjected to very high tensileloads. The strength of the prosthesis must in some cases by many timesthat of the peak load to which it will be exposed to compensate for themechanical properties of the prosthesis which are time-dependent.

From a mechanical strength standpoint, one of ordinary skill in the artwould realize that the number of individual strands needed for aparticular application will depend on several factors. These include:the individual strand cross-sectional area; the tensile strength of theindividual strand; and the tensile force requirement for that particularapplication, including any safety factors for creep strain limitations.The individual strands used in this invention can be constructed usingthe processes described in U.S. Pat. No. 3,953,566, U.S. Pat. No.3,962,153 or following Example B. It is desirable to use a high matrixtensile strength material in order to minimize the overall physicaldimensions of the device and thereby minimize the size of drill holesplaced in the bone to mount the device. Matrix tensile strength refersto the strength of the polymer in a porous specimen and is used asdefined in U.S. Pat. No. 3,953,566.

In the preferred form of this invention:

The strand material is porous PTFE with a matrix tensile strengthgreater than 20,000 psi, a porosity greater than 30%, and amicrostructure characterized by intercommunicating pathways formed bythe boundaries of nodes and fibrils.

Strand dimensions and characteristic interstitial dimensions of themicrostructure are selected such that tissue ingrowth throughout thestrand takes place in a rapid fashion.

Each strand and the finished construction possess sufficient strengthnecessary to meet the mechanical requirements of the particularapplication.

The parallel strands result from multiple loops formed from a continuousfilament of the strand material.

The ends of the multiple loops are gathered and formed into at least oneeyelet for attaching the article to bone tissue.

The uniformity of strand loading of the prosthesis under tensile forceis enhanced through:

1. Minimizing differences in loop length used to form the parallelstrands.

2. Compression of the loop strands in the eyelet segments to providestrand-to-strand adhesion.

The prosthesis also includes means for distributing the tensile loadamong the strands as it passes around a radius, said means including:

1. A twist in the strand bundle about its longitudinal axis.

2. A loose strand braid.

Although the ligament prosthesis embodiment in FIG. 10 is shown with apair of opposing eyelets formed from elongated loops, the presentinvention also encompasses a single eyelet 324 formed in the loopsgathered for attachment to bone. The loops at the other end 316 remainungathered or splayed to provide attachment to soft tissue such asmuscle tissue, as by suturing (see FIG. 5). In this latter case, theclosed loop ends provide additional resistance to possible strandslippage past the sutures. The single eyelet embodiment of thisprosthesis 310, could find use in the repair or replacement of tendons.

EXAMPLE A

This example demonstrates a prosthetic device which did not achievesatisfactory system strength because the strand thickness was too largefor the interstitial dimension which characterized its microstructure(FIG. 1). The strand thickness (diameter) was 0.26 inches, porosity ofthe strand was approximately 80%, and the characteristic interstitialdimension was about 78 microns. This interstitial dimension wasdetermined as shown in FIG. 2. The prosthesis was used to replace theanterior cruciate ligament of a dog by routing the material throughdrill holes in the tibia and femur. Four holes were drilled in the tibia2 and femur 4 such that the prosthesis strand 6 formed a loop ofmaterial with two strands in the position of the original ligament (FIG.6). Initial fixation was provided by tying the ends of the strandtogether in a knot 8 to form a continuous loop. Ingrowth and formationof tissue within the interstices of the microporous material wereexpected to augment the initial fixation strength and to distributestresses to the surrounding tissue. Each of the strands crossing theknee joint possessed a tensile strength of about 550 pounds. Thecombined strength of these two strands was then 1,100 pounds. Afterhaving been implanted for 260 days, the knee joint was explanted.

Drill holes were placed in the tibia and femur for mounting into tensiletest clamps. After removal of all supporting collateral structures aboutthe knee, the femur was distracted from the tibia along the axis of theprosthetic ligament at a constant rate of 500 mm per minute untilfailure. The length spanning the intra-articular space between bonetunnels represented that portion of the prosthesis placed under tensileload during the test, due to tissue attachment to the prosthesis in thebone tunnels. The failure mode of the system was rupture of theprosthetic device at the level of exit from the bone tunnels.Surprisingly, this rupture took place at a value of only 200 lbs.Through histological inspection, we discovered that this reduction instrength was related to the restriction of bony ingrowth to generallyless than 1 mm depth into the prosthesis. With a strand of this diameterand characteristic interstitial dimension, attachment takes place onlyat a circumferential ring of material on the periphery of the device.This reduced area then becomes the only load-bearing material of theprosthesis as a tensile force is initially applied. Failure occurs inthis circumferential ring of material first and then progresses throughthe central portion of the prosthesis.

EXAMPLE B

The experience cited in Example A led to the discovery that tissueingrowth must penetrate throughout the cross-section of the strand inorder to provide adequate long-term system strength. Accordingly, adevice was constructed using a strand of similar porosity andcharacteristic interstitial dimension but with a much smaller diameter.The strand material used to construct the anterior cruciate ligamentprosthesis of this example was made as follows:

PTFE dispersion powder ("Fluon CD 123" resin produced by ICI America)was bended with 130 cc of "ISOPAR K" odorless solvent (produced by ExxonCorporation) per pound of PTFE, compressed into a pellet, and extrudedinto a 0.108 inch diameter rod in a ram extruder having a 96:1 reductionratio in a cross-section from the pellet to the extruded rod.

The extruded rod still containing Isopar K was immersed in a containerof Isopar K at 60° C. and stretched to 8.7 times its original lengthbetween capstans with an output velocity of about 86.4 ft/min. Thesecapstans were about 2.8 inches in diameter with a center-to-centerdistance of about 4.5 inches. The diameter of the rod was reduced fromabout 0.108 inch to about 0.047 inch by this stretching. The Isopar Kwas then removed from this stretched material.

The stretched rod was then pulled through a circular densification dieheated to 300° C. The opening in the die tapered at a 10° angle fromabout 0.050 inch to 0.025 inch and then was constant for about 0.025inch length. The output velocity of the material exiting the die was 7.2ft/min.

The stretched rod was then heated to 300° C. through contact withheated, driven capstans and stretched 41/2 fold (350%) with an outputvelocity of 6.5 ft/min. These capstans had a diameter of 2.75 inches anda center-to-center distance of 4.5 inches.

Finally, the rod was restrained from shrinking and exposed to about 367°C. in an air oven for 30 seconds.

In the finished form, the fiber made with this process possessed thefollowing characteristics:

Diameter=0.026 inches

Matrix Tensile Strength=74,000 psi

Porosity=80.8%

Characteristic Interstitial Dimension=74 μ

As illustrated in FIG. 7, prosthesis 10 was constructed on two steelspools 42, 44 which were mounted on a rack (not shown). The spools weresupported on studs 46, 48 spaced 14 cm from center line to center line.These steel spools were threaded to allow demounting of one flange. Thestrand of PTFE material was passed around these two spools 80 times sothat a total of 160 strands connected the two spools. The two free endsof the fiber were tied together with multiple square knots. One spoolwas demounted from the stud, rotated through 180° and remounted on thestud, thus imparting a one-half twist about the longitudinal axis of theconstruction. The construction was then wrapped with a thin film of PTFEa total of 25 revolutions each at three locations. This film wasmanufactured according to the teachings of U.S. Pat. No. 3,962,153 andhad the following characteristics:

Width=0.375"

Thickness=0.0025"

Longitudinal matrix tensile strength=70,000 psi

Porosity=84%

The bundle of strands was wrapped with this thin film at two points 28,30 adjacent to the spools 42, 44, thereby forming eyelets 24, 26, at theends of the construction (FIG. 8). A central portion 38 was also wrappedwith film. The two spools were then demounted from the studs and placedon a rack constructed of thin metal wire designed to prevent rotationand longitudinal shrinkage. This rack was then exposed to 375° C. in anair oven for six minutes. After cooling, the spools were demounted fromthe ends of the construction. The position occupied by the spoolsprovided eyelets through which this ligament prosthesis construction canbe attached to bone with screws or other suitable means of fixation. Allareas which had been wrapped with film had become compressed during theheating treatment due to film shrinkage, thereby providingstrand-to-strand cohesion. During the previously described heatingcycle, some fiber-to-fiber attachment in the unwrapped regions also tookplace. These fibers were then individually separated using a metal pick.The construction then comprised 160 microporous PTFE strands connectingtwo eyelets of somewhat densified material. Prosthesis 10 included a180° twist along the tensile load direction to better distribute thetensile load among strands 20. PTFE tape wrap 38 surrounding strands 20and positioned approximately midway between ends 14, 16 of prosthesis 10serves to maintain the twist by securing strands 20 against untwistingduring implantation. As with PTFE wraps 28, 30, wrap 38 is intended tobe positioned outside of the bone contact area so as not to inhibittissue ingrowth into strands 20.

A device prepared in the manner just described was implanted into theknee of a sheep to replace the excised host anterior cruciate ligament(see FIGS. 11A, B and C). This implantation was accomplished through theplacement of one 1/4" drill hole in both the tibia and femur. Theplacement of the hole in the tibia was along the axis of the previouslyremoved natural anterior cruciate and exited at the insertion site. Theplacement of the femoral drill hole began at the lateral distal femoralsurface proximal to the femoral epicondyle. The tunnel was angled suchthat the exit hole was created just proximal to the lateral femoralcondyle on the popliteal surface of the femur. The prosthesis 10 wasrouted from the femoral exit site through the intercondylar space,across the intra-articular space, and through the tibial tunnel. Theeyelets 24, 26 and wrapped segments 28, 30 at the ends of theconstruction were positioned to be to the outside of the drilled bonetunnels. The placement of the wrapped segment 38 at the center region ofthe construction was in the intra-articular space. The prosthesis 10 wasthen anchored to bone with self-tapping orthopedic screws 32, 34 placedthrough the eyelets 24, 26. The knee joint was determined to be stableimmediately after the operation.

After three months implant time, the knee was removed from the animaland drill holes placed in the tibia and femur into which clamps weremounted to provide for tensile testing along the axis of the ligamentconstruction. After removal of muscle tissue and severing of alsupporting collateral structures about the knee, the femur wasdistracted from the tibia at a constant rate of 500 mm per minute untilfailure. System failure took place at 642 lb. The failure took place inthe ligament prosthesis at the eyelet secured to the femur. Rupture tookplace as the load exceeded the fixation provided by tissue ingrowth intothe intra-osseous segments and was transferred to the fixation screw.Device failure was related to an unwinding of the strand materialthrough the eyelet segments after several strands had failed. Histologicinspection of this sample showed tissue ingrowth among and into thestrands. Tissue ingrowth had proceeded completely through the diameterof some strands. We anticipate that with longer implant times themajority of strands would have shown complete and thorough ingrowth.

What is claimed is:
 1. Method for making a tensile load-bearing tissueprosthesis having a longitudinal axis parallel to a plurality oflongitudinally adjacent strands connected to at least one eyelet forinitially attaching the prosthesis to tensile force-applying bonetissue, the method comprising the steps of:a. selecting a strandmaterial capable of permitting tissue growth in and through the strandmaterial; b. arranging and spacing a plurality of pin means to define anelongated area in accordance with the desired size of the prosthesis; c.forming a plurality of elongated loops around the plurality of pin meansfrom a continuous filament of the selected strand material until adesired number of parallel strands is obtained; and d. gathering theloop ends at one end of the elongated area to form the at least oneeyelet, said gathering step including the step of securing the gatheredloop ends against ungathering.
 2. Method as in claim 1 including theadditional step of imparting a twist to the loops about the longitudinalaxis of the prosthesis after said step of forming a plurality ofelongated loops.
 3. Method as in claim 2 wherein the loops are twistedabout 180°.
 4. Method as in claim 1 wherein said step of forming aplurality of elongated loops includes the step of loosely braiding thestrand material.
 5. Method for making a tensile load-bearing tissueprosthesis having a longitudinal axis parallel to a plurality oflongitudinally adjacent strands connected to at least one eyelet forinitially attaching the prosthesis to tensile force-applying bonetissue, the method comprising the steps of:a. arranging and spacing aplurality of pin means to define an elongated area in accordance withthe desired size of the prosthesis; b. forming a plurality of elongatedloops around the plurality of pin means form a continuous filament ofthe desired strand material until a desired number of parallel strandsis obtained; and c. gathering the loop ends at one end of the elongatedarea to form the eyelet, said gathering step including the step ofsecuring the gathered loop ends against ungathering;wherein the securingagainst ungathering step includes the step of compressing and heatingthe gathered ends at a temperature and for a time sufficient to coalescethe loop ends.
 6. Method as in claim 5 wherein said step of securingagainst ungathering also includes the preliminary step of wrappingstrands of a high strength material about the gathered ends in adirection orthogonal to the prosthesis axis at a position adjacent tothe eyelet.
 7. Method as in claim 6 wherein the high strength materialis expanded polytetrafluoroethylene having a matrix tensile strength ofabout 74,000 psi.
 8. Method as in claim 5 wherein the gathered ends arecompressed and heated in a die having the desired eyelet shape anddimensions.
 9. Method as in claim 1 wherein said step of forming aplurality of elongated loops includes the step of imparting a twist tothe continuous filament.